As one example of conventional power conversion circuits in which a freewheeling diode is connected anti-parallel to a switching element of a main circuit, there is known a power conversion circuit in which a semiconductor switch corresponding to one arm of a bridge circuit is configured by serially connecting an auxiliary element and a main element, which are respectively formed of a voltage-driven switching element and an anti-parallel diode. Further, a high-speed freewheeling diode with a withstanding voltage equivalent to that of the main element is connected anti-parallel to the serial connection of the main element and the auxiliary element.
In such a power conversion circuit as noted above, respective main elements of the upper and lower arms of the bridge circuit are supplied with an off-signal and come into a gate-off state if a device is abnormally stopped due to such causes as a load current becoming too high or a device temperature increasing to a specified value or more. At this time, the auxiliary element is also continuously supplied with the off-signal in synchronism with the main element. This makes it difficult to feed an electric current to the auxiliary element of the respective arms. Thus, a main current continues to flow through the high-speed freewheeling diode. The current flowing in the gate-off state shows a sharp increase in its average value as compared with a normal operation in which an electric current flows only during a time interval between the off-time of one semiconductor switch and the on-time of the other semiconductor switch in a single-phase equivalent bridge circuit, i.e., during the dead time interval. This is accompanied by a significant increase in the loss, i.e., the amount of heat generated in the high-speed freewheeling diode. Therefore, a problem is posed in that the high-speed freewheeling diode requires a stricter cooling design. Although a SiC diode is attracting attention as one example of high-speed diodes in recent years, the SiC diode remains expensive. Accordingly, there is a need for a low-withstanding voltage low-capacity SiC diode and a circuit implementation including the same that can be realized with a relatively low price.